Fast pyrolysis of biomass is a thermochemical conversion process that provides an economic production of pyrolysis oils/bio-oils. The process also results in a residual solid residue, char, that comprises carbon and mineral ash that can be a potential source of fuel or a valuable co-product. Depending on the exposure time and temperature, pyrolysis can increase the interfacial surface areas of the residual char thereby enhancing its absorptive capacity. Char residues can be used for physical or chemical absorption and as catalyst support or base material for fertilizers. The reactions that occur during char combustion or gasification are heterogeneous hence the reaction rates are microstructure dependent. Ashes from biomass derived chars can be high either in calcium or silica with the latter exceeding 90% levels in certain grain hull residues. Depending on the microstructural transformations which occur during thermal degradation of the biomass, silica-laden ashes can be a potential source of pozzolan for the construction industry. In this study, the microstructure of the chars derived from fast pyrolysis of barley-hull was studied using environmental scanning electron microscopy under low vacuum conditions. The results indicate a gradual increase in convoluted microstructure related to the superficial organization of epidermal cells, including stomata and trichomes that eventually assume the form of various morphotypes of phytoliths. Characterization of the temporal events of high temperature evolution of the hull microstructure provides practical implications of its combustion reactivities and also provides information useful for predicting potential masonry applications for the resulting ash. (c) 2006 Elsevier Ltd. All rights reserved.